In the process of fabricating semiconductor devices, very thin films are used to form dielectric gates. Currently, silicon dioxide is the most common material used for the gate dielectric. With the push towards smaller devices, thinner gate dielectric layers are needed. Today, these layers are only 10 to 20 Angstroms thick. To obtain the necessary characteristics with these very thin layers, the industry is moving towards adding nitrogen to the silicon dioxide material.
The amount of nitrogen added to the silicon dioxide must be accurately controlled and therefore a precise method for measuring nitrogen concentration is required. Metrology efforts in the past have focused upon secondary ion mass spectrometry (SIMS) and X-Ray type measurements such as ESCA (Electron Spectroscopy for Chemical Analysis) or XPS (X-ray Photoemission Spectroscopy). Attempts have also been made to characterize nitrogen levels using either ellipsometry or spectroscopy.
It is believed that neither spectroscopy nor ellipsometry alone can provide sufficient information about nitrogen levels in a sample. Therefore, it would be desirable to develop one or more approaches for monitoring nitrogen levels that was fast, accurate and non-destructive.